Comprehensive nuclear materials 3 16 ceramic fuel–cladding interaction

Comprehensive nuclear materials 3 16 ceramic fuel–cladding interaction Comprehensive nuclear materials 3 16 ceramic fuel–cladding interaction Comprehensive nuclear materials 3 16 ceramic fuel–cladding interaction Comprehensive nuclear materials 3 16 ceramic fuel–cladding interaction Comprehensive nuclear materials 3 16 ceramic fuel–cladding interaction Comprehensive nuclear materials 3 16 ceramic fuel–cladding interaction

K Maeda

Japan Atomic Energy Agency, O-arai, Ibaraki, Japan

ß 2012 Elsevier Ltd All rights reserved.

3.16.1 Introduction and Overview of Ceramic Fuel–Cladding Interaction 444

3.16.2.1 Formation of Protective Oxides on Cladding Materials 4453.16.2.2 Chemical Interaction Among Oxide Fuels, FPs, and Cladding 446

3.16.4 Occurrence of Interaction Between Oxide Fuels and Cladding 449

3.16.4.1.4 Effect of temperature difference between fuel and cladding 452

3.16.6 Inhibition Methods for Oxide Fuel and Cladding Interaction 466

Abbreviations

AISI American Iron and Steel Institute

ANL Argonne National Laboratory

CCCT Cladding component chemical transport C/M Carbon-to-metal

DFR Dounreay fast reactor

443

EBR-II Experimental breeder reactor-II

EPMA Electron probe microanalysis

FAE Fuel adjacency effect

FCCI Fuel and cladding chemical interaction

FCMI Fuel–cladding mechanical interaction

FFTF Fast flux test facility

FP Fission product

FPLME Fission product-induced liquid-metal

embrittlement

FR Fast reactor

GE General Electric Company

HEDL Hanford Engineering Development

Laboratory

JOG Joint oxide-gaine (French)

LWR Light water reactor

MOX Mixed oxide

MX Nonoxide, where M stands for U þ Pu and

X stands for C or N

NMA Nuclear microprobe analysis

N/M Nitrogen-to-metal

O/M Oxygen-to-metal

PFR Prototype fast reactor

PIE Postirradiation examination

PNC Power Reactor and Nuclear Fuel

Development Corporation, currently

Japan Atomic Energy Agency

RIFF Re´action a` l’Interface Fissile Fertile (French)

SIMS Secondary ion mass spectrometer

SNR Schneller Natriumgeku¨hlter Reactor

(German)

3.16.1 Introduction and Overview of

Ceramic Fuel–Cladding Interaction

Ceramic fuels used for fast reactors (FRs) are oxide

fuels and nonoxide ceramic (MX-type, where M stands

for Uþ Pu and X stands for C and N) fuels such as

carbide and nitride fuels Ceramic fuel–cladding

inter-actions (FCCI, fuel and cladding chemical interaction)

are mainly divided according to oxide and MX-type

fuels FCCI is more complicated in oxide fuels than

in MX-type fuels because oxide fuels lead to oxidation

of cladding materials and formation of various oxides

of fission products (FPs) by irradiation

Chemical interactions between uranium and

plu-tonium mixed oxide (MOX) fuels and/or FPs and

cladding materials are considered as one of the major

factors limiting the lifetime of fuel pins in FRs This

limitation is especially important for long-term

irra-diation of (U,Pu)O fuel pins clad in stainless steel

Fuel pins for FRs are generally designed with Type

316 stainless steel cladding to operate with a peakcladding hot-spot temperature of 700C

Results and analyses of irradiation experimentsrelated to FCCI with oxide fuels have been reported

in various technical society conferences and topicaland periodic reports since FCCI was first reported inthe late 1960s FCCI is nowadays recognized as one

of the major factors determining integrity and time of oxide fuel pins as demonstrated in numerousin-pile and out-of-pile tests Some mechanisms ofcladding attack have been proposed from the results

life-of the many postirradiation investigations and dynamic analyses of the postulated chemical reactions.Cladding and oxide fuels do not violently react,even under a high oxygen potential condition; theyonly form a protective layer on the inner wall ofthe cladding But when fuel pins are irradiated in areactor, the additional effect of the generated FPsinduces cladding attack A number of experimentshave shown that both stoichiometric and hypostoi-chiometric oxide fuels react with stainless steel clad-ding when irradiated in typical FRs On the otherhand, out-of-pile tests between (U,Pu)O2 or (U,Pu)

thermo-O2xand several stainless steels have shown that nodetectable reaction took place within the times andtemperatures of interest for FRs When hyperstoichio-metric fuel, (U,Pu)O2 þ x, was tested, cladding attackwas detected and the difference in reaction behaviorwas ascribed to the excess oxygen provided in thehyperstoichiometric fuel Inability to reasonablyextrapolate the out-of-pile results to the in-pile results

is of concern for the design of oxide fuel pins, and itindicates that the prediction of lifetime is complicated.The thermochemistry of the fuel–cladding gap iscomplex as well and difficult to predict because itdepends not only on concentrations of corrosive FPs,but also on major parameters such as fuel–claddinggap width, fuel oxygen-to-metal (O/M) ratio, claddingtemperature, fuel temperature, and radial temperaturegradient FCCI is further regarded as sensitive tolinear heat rating and likely to change with fuelburnup When the swelling of the cladding is highand the fuel–cladding temperature gap is large, theprobability of attack is enhanced Thus, cladding attacktends to be unpredictable, and it may be locally worsecompared to the overall condition The possible con-sequence is complete penetration of the cladding by achemical mechanism alone In addition to this, it may

be considered that there is some form of stress sion cracking in the cladding Actual creep strain onthe cladding is from fuel and cladding mechanical

corro-interaction (FCMI) and/or internal FP gas pressure.

FCCI has been identified as a contributing factor in

the breaches of oxide fuel pins Observations at an

axial location of a breach that was located at the

approximate original top of the fuel column have

shown extensive FCCI The breach was a consequence

of FCMI and internal FP gas pressure

As explanations of the observed effects of FCCI

have been speculative, fuel pin design could rest

only on empirical equations rather than on

fundamen-tal models Cladding wastage equations by FCCI have

been developed for fuel pin designs Most observations

of FCCI showed it to be the result of simple oxidation

of the inner surface of the cladding Three principal

types of cladding attack in stainless steel can be

distin-guished The first is a general oxidation of the inner

surface of the cladding The second is intergranular

attack and is the most important The third is advanced

attack which appears to be a transport of the cladding

constituents into the fuel It is typically seen as wastage

of the cladding thickness in some local areas by

mechan-ical or liquid-phase transport of cladding constituents

into the outermost oxide layer on the fuel pellets

FCCI with oxide fuels has been recognized as an

important factor in the ability to achieve peak

burn-ups in the range of 10 at.% in FRs while maintaining

high coolant bulk outlet temperatures However, in

addition to cladding thickness losses due to FCCI,

oxide fuels and FPs have the potential for reducing

cladding load-bearing capabilities by mechanisms such

as liquid-metal embrittlement (FPLME, FP-induced

liquid-metal embrittlement)

The other type of FCCI occurs for nonoxide

ceramic (MX-type) fuels such as carbide and nitride

fuels and the cladding MX-type fuels are chemically

unreactive to sodium coolant, so sodium may also

pos-sibly be used as a medium for bonding between fuel and

cladding instead of helium gas MX-type fuels are

generally irradiated at lower temperatures and lower

radial temperature gradients than oxide fuels, although

at high linear heat rating, which results in low FPs

release rate The volatile FPs (Br, I, Cs, and Rb) do

not form carbides or nitrides In particular, MX-type

fuel pins are kept with low oxygen potential at the inner

cladding surface; therefore, severe oxidative FCCI of

the FPs is not expected A number of irradiation

experi-ments have been performed with MX-type fuels to

study FCCI The compatibility with cladding materials

has been investigated in out-of-pile examinations and

thermodynamic analyses As a consequence, unlike the

case of oxide fuels, FPs from MX-type fuels do not play

a major role in FCCI Instead, the carburizing and

nitriding of cladding, and also the formation of metallic compounds of fuel and cladding, have beeninvestigated as a major FCCI of MX-type fuels.The carbide fuels (U,Pu)C, which are designed to

inter-be slightly hyperstoichiometric, will therefore inter-be

in the two-phase region (U,Pu)Cþ (U,Pu)2C3 Thepresence of higher carbide phases carburizes thecladdings Hyperstoichiometric carbides can embrit-tle the cladding by forming grain boundary carbideswhich can lead to intragranular failure of the steelafter a moderate burnup The creep and swellingproperties of stainless steels are sensitive to carbur-ization and precipitation of M23C6 As sodium can act

as a transfer agent, carbon transport rates through thegap in sodium-bonded fuel greatly increase Hypos-toichiometric mixed carbides contain (U,Pu) metal as

a second phase which may form low melting-pointeutectics with iron or nickel base cladding alloys.Hyperstoichiometric MN1 þ x-containing sesqui-nitride phase can cause nitrogen penetration andform a reaction layer at the cladding inner surface,which results in the clad nitriding The nitriding

of cladding generally decreases the ductility andincreases the mechanical strength Hypostoichio-metric MN1  xcontains free metal leading to a eutec-tic melting reaction between the free (U,Pu) metal andthe cladding, which results in formation of (U,Pu)Fe2-and (U,Pu)Ni5-type intermetallic compounds

At present, it is clear that the knowledge base forMX-type fuels is much smaller and less detailed thanthat for oxide fuels, and addition to the base is a work

in progress However, MX-type fuels merit much lessconcern regarding cladding–fuel compatibility thanoxide fuels

In Sections 3.16.2–3.16.6, the causes of FCCIwith oxide fuels are reviewed, considering the depen-dence on irradiation conditions and fuel parameters

as well as types of cladding material Furthermore, therole of corrosive FPs in the FCCI, the mechanism ofFCCI, and the FCCI enhancement by oxygen poten-tial are summarized The MX-type fuel–claddinginteraction is briefly described inSection 3.16.7

3.16.2 Cladding Compatibility with Oxide Fuels and FPs

3.16.2.1 Formation of Protective Oxides onCladding Materials

Out-of-pile tests between (U,Pu)O2 or (U,Pu)O2  xand several stainless steels have shown that nodetectable reaction took place for exposure times

and temperatures of a typical FR However, in

hyper-stoichiometric fuel, (U,Pu)O2 þ x, cladding attack was

detected Excess oxygen was provided by the

hyper-stoichiometric fuel, which was considered to cause

the difference in the reaction behavior Therefore, if

the fuel surface O/M ratio can be maintained just

below exact stoichiometry, oxidation of the cladding

cannot take place

Of the three major constituents of austenitic

stain-less steel cladding, Fe, Cr, and Ni, chromium has

the greatest affinity for oxygen and forms the most

stable oxide Initially, chromium begins to get

oxi-dized when the oxygen partial pressure satisfies the

equilibrium condition of the reaction

 4=3CrðcladdingÞ þ O2ðgÞ ¼ 2=3Cr2O3ðsÞ;

where the oxygen potentialDGO2ð¼ RTlnpO2Þ of the

fuel surface reaches554 kJ mol1at 727C.1

How-ever, fuel and cladding do not severely react, even

when the oxygen potential is high; they only form a

protective layer on the inner wall of the cladding The

stable protective Cr2O3thin layer prevents the fuel

and cladding reaction from becoming

thermochemi-cally equilibrated

In the initial stage of irradiation, the oxygen

potential of the fuel surface rises because of oxygen

redistribution Excess oxygen, after uranium and

plu-tonium fission in the fuel, leads to an increase in fuel

O/M ratio with burnup Radial redistribution of

oxy-gen along the fuel radial temperature gradient

enhances the increase of O/M ratio at the fuel

sur-face It appears unlikely that the oxygen potential at

the entire fuel–cladding interface can be kept low

enough to prevent cladding oxidation throughout

the entire lifetime of the fuel element Thus, a thin

protective layer of oxide, mainly Cr2O3, soon forms

on the inside surface of the cladding, thereby

physi-cally separating the substrate metal from the

oxidiz-ing medium Further growth of this layer requires

that chromium ions diffuse from the substrate metal

to the outer surface of the coating or that oxygen ions

migrate in the opposite direction The rates of both

these processes are very slow at 727C because of

the low values of the diffusion coefficients of the ions

in the oxide layer If the thermochemically stable

uniform layer is breached by mechanical forces or is

dissolved by a component of the oxidizing

environ-ment, the substrate metal is exposed to rapid attack

The integrity of the cladding relies on the kinetics

of the chemical attack in an environment where

oxidation is thermodynamically possible In addition,

the inner wall temperature of the cladding in

an FR-MOX fuel pin reaches the range at which

the sensitization of stainless steel cladding occurs,

500C.2

This suggests that the corrosion resistance

of the stainless steel cladding might become degradedbecause of chromium being held in carbide particles

in the cladding

Oxide Fuels, FPs, and Cladding

As long as the protective layer stays intact, the stainlesssteel cladding is protected from further corrosion.However, a number of irradiation experiments haveshown that both stoichiometric and hypostoichiometricfuels reacted with stainless steel cladding Unlike irra-diated fuel, fresh fuel does not corrode stainless steelcladding to the same extent It was suggested thatirradiation damage might reduce the effectiveness ofthis protective layer But it was found that the extent

of oxidation did not sufficiently increase whileirradiation damage by fission fragments increased.3The thermodynamic tendency of oxide fuels is tooxidize the cladding, and not to violently attack it

in the absence of FPs because of the protectionprovided by the oxide film formed on the surface ofthe steel.4However, the protective layer is impaired

by a chemical reaction of reactive FPs and oxygenwith chromic oxide Such evidence suggests that one

or more of the FPs are responsible for acceleratingthe chemical reactions between fuel and cladding inirradiated fuel pins.3

FCCI is the FP-accelerated oxidative attack of thecladding that is frequently observed in FR fuel pinsinvolving reactive FPs such as Cs, Te, and I.4Specifi-cally, cesium and tellurium are thought to contribute

to the most aggressive intergranular attack modes.4,5The FCCI phenomenon is generally recognized to bethe result of the oxidation of chromium in the stain-less steel cladding under the influence of FPs; clad-ding attack by Cs2Te has not been considered as anoxidation mechanism of the cladding materials

In irradiation experiments, however, the protectiveoxide layer is breached in some places and claddingattack takes place, usually in a few isolated patchesrather than uniformly Whether a chemical reactionbetween components of the irradiated fuel and consti-tuents of the cladding can occur at all is determined bythe thermodynamics of the reactions involved Localbreakdown of the protective layer and subsequentcorrosion appear to depend on the local accumulation

of observed major FPs, such as Cs and Te or I, whichare considered important corrosive elements

The generated volatile FPs are released and mulate at the fuel–cladding gap with increasing

accu-burnup When fuel surface oxygen potential exceeds

the threshold necessary for oxygen transport to the

cladding inner surface, excess oxygen and corrosive

FPs can interact with the cladding inner surface leading

to FCCI Internal wastage of the stainless steel cladding

is related to the complex phenomenon of corrosion

established by the presence of FPs (Cs, I, and Te) and

oxygen at the fuel–cladding interface The threshold

temperature for cladding attack is around 500C

3.16.3 Morphology of Cladding

Attack in Oxide Fuel Pins

3.16.3.1 Observations of Cladding Attack

3.16.3.1.1 Deep localized cladding attack

Regions of chemical reactions between the fuel and

cladding have been generally observed, especially the

hotter cladding temperature regions Examination of

metallography samples showed occurrence of

non-uniform and deep localized cladding attack in

irra-diated fuel pins.6,7Cladding attack usually occurred

in an irregular manner over the inner surface of the

cladding and in the case of intergranular attack, its

depth of penetration varied from site to site In

addi-tion, the observed deep localized interaction was

usually of a different type than in the rest of the

sample When access to the substrate metal was

estab-lished, cladding attack by FPs occurred, either

uni-formly or only locally, but in some cases it penetrated

more than 100mm into the cladding This would be a

significant reduction of the effective thickness of the

cladding Despite the reduction in cladding thickness,

actual fuel pin failure has rarely been observed

The occurrence of a deep localized interaction of

more than 100mm in depth was observed in a sample

which had an initial O/M ratio larger than 1.98, and

was irradiated to less than 5 at.% burnup at cladding

temperature higher than 650C.6That suggests that

this type of interaction occurs primarily in

high-temperature regions with relatively low burnup

This interaction is called deep localized FCCI

and is an intergranular type of cladding attack,

char-acterized by a highly localized reaction product

Because cladding attack tends to be random, it becomes

locally worse compared to the overall condition For a

fuel with an initial O/M ratio of 1.99–2.00, there is

evidence that intergranular attack of sensitized stainless

steel cladding occurs in the matrix around the carbide

particles in the grain boundaries.8Microprobe

exam-inations have shown this area to be depleted in

chro-mium and manganese, with significant quantities of the

FP cesium present in the reaction product This was

considered as intergranular corrosion accelerated bysensitization which is seen as the loss of Cr by Cr23C6precipitation in the grain boundary.9 As this was themost aggressive form of FCCI observed along grainboundaries deep in the cladding in the case of initialO/M ratios above 1.98, this type of attack has beenlargely eliminated by using fuel with O/M ratios of 1.98and below By utilizing an appropriate lower O/M fuelassociated with longer irradiation for excess oxygen

in the fuel pin, a more uniform matrix interactiontends to take place

A combined interaction form, consisting of matrixFCCI proceeded by intergranular FCCI, occurs infuel with moderate O/M ratio and high burnup.23.16.3.1.2 FCCI at the top of the fuel columnThe top of the fuel column at a cladding temperaturenear the maximum corresponds to the boundary offissile–fertile fuel pellets At this location, axiallymigrated and accumulated volatile FPs react withthe cladding material Axial isotopic gamma scansfor high burnup pins have shown that there are largeramounts of cesium in the area of the upper insulatorpellets than in the area of the fuel.10Because of themigration of cesium to the cold region in the irra-diated fuel pins, cesium peaks are generally found atboth ends of the fuel column

These accumulations were generally related to theformation of a phase consisting of U–Cs–O (Cs2UO4)

at the UO2blanket or insulator pellets, which causedlocalized inelastic deformations of the cladding (up to

30mm) at the fuel–blanket interfaces by a volumetricchange.10,11 But Kleykamp12 confirmed that instead

of Cs2UO4, a cesium uranoplutonate Cs2(U,Pu)4O12layer was formed on the grain boundaries of the

UO2 blanket pellets in the irradiation experiment.Furthermore, formation of compounds at the UO2blanket or insulator pellets led to a severe intergran-ular attack of the cladding (up to 100mm) in thisregion.10,11Figure 1shows ceramographs for a longi-tudinal section removed from a fuel pin (maximumburnup 14.5 at.%, 695C cladding inner surfacetemperature, and initial O/M ratio 1.984).6 Boththe depth and character of the FCCI had changedsignificantly at the fissile–fertile transition zone.The maximum depth of cladding attack at the fertileand fissile fuel pellets was approximately 90 and

135mm, respectively A similar localized form of ding attack occurred at higher temperatures of

clad->600C at the fissile–fertile interface.13

This fissile–fertile interface reaction, termed RIFF (Re´action a`l’Interface Fissile Fertile, in French), is associatedwith migration of volatile FPs to the end of the fuel

column There is no evidence of RIFF in PE-16 (high

Ni alloy) and EM-12 (ferittic–martensitic steel alloy)

The occurrence of RIFF appears to depend on the

choice of cladding materials.13

The change in character of the cladding attack at the

top of the fuel column suggests a change in the

mecha-nism of chemical interactions at locations of fissile and

fertile fuel pellets The reaction of fuel and cesium

suggests the presence of a high oxygen potential in

the fuel.14Large cesium pressures, which are generally

expected in hypostoichiometric fuel, lead to the

forma-tion of cesium uranate in the UO2blanket or insulator

pellets The FP inventory and the radial temperature

gradient in the region of the UO2blanket or insulatorpellets are significantly different from those at theregion of the fissile fuel column The predominantlyradial heat transfer in the upper region of the fuelcolumn and the absence of heat-generating material

in the UO2blanket or insulator pellet suggest little or

no thermal gradient across the UO2-cladding gap.This effect has been taken into account in the design

of other irradiation experiments by reducing the volume

of the blanket pellets.15The depth of chemical tion between cladding and fuel outside the blanket–fuelinterface has always been lower than 60mm

interac-3.16.3.2 Types and Characteristics ofCladding Attack

From the metallographic examination of stainlesssteel–clad fuel pins irradiated to various burnup levels,

it is generally possible to observe the character of theevolving cladding attack along the cladding tempera-ture distribution in the fuel pin The cladding attack

is classified into three types: (1) matrix, (2) ular, and (3) combined matrix and intergranular (alsocalled ‘advanced’ or ‘evolved’).16Figure 2(a) and 2(b)show typical photomicrographs of matrix and inter-granular types of attack in a fuel pin with Type 316stainless steel cladding (burnup: 50 MWd kg1 M),respectively In addition,Figure 2(c)shows a severecombined intergranular and matrix attack observed

intergran-in the fuel pintergran-in with Type 347 staintergran-inless steel ding (burnup: 140 MWd kg1M)

clad-The first type of cladding attack is a general dation and is confined mainly to the shallow innersurface of the cladding The entire body of the innerwall of the cladding is converted to a reaction zonecontaining the oxides of Fe, Cr, and Ni In the regions

oxi-of matrix attack, EPMA (electron probe sis) results show a depletion of iron and nickel and

microanaly-UO 2 (U,Pu)O2

Figure 1 Cladding attack in the vicinity of the fissile–fertile

fuel interface Reproduced from Lawrence, L A Nucl.

Technol 1984, 64, 139–153, with permission from ANS.

Figure 2 Types of representative cladding attack: (a) matrix, (b) intergranular, and (c) combined or evolved Reproduced from Perry, K J.; Melde, G F.; McCarthy, W H.; Duncan, R N., In Fast Reactor Fuel Element Technology, Proceedings of Conference, New Orleans, Luisiana, Apr 13–15, 1971; Farmakes, R., Ed.; American Nuclear Society: Hinsdale, IL, 1971;

pp 411–429, with permission from ANS.

an enhancement of chromium and cesium Trace

amounts of iodine and tellurium are also observed in

the region of matrix attack.16When this type of

clad-ding attack evolves, there is a definite segregation of

the cladding constituents in the reaction product layer

The reaction product of the matrix attack is a mixture

of metal particles and nonmetallic compounds in the

fuel–cladding gap In addition to the three major

con-stituents of the cladding, the reaction zone contains the

FPs (Cs, Mo, and lesser amounts of I, Te, and Pd) The

reaction zone does not appear to contain the heavy

metals (U and Pu), and neither does the cladding

The attack on the grains is uniform with no strong

preference for attack along the grain boundaries

The second type of cladding attack is penetrating

the stainless steel cladding along grain boundaries

and it is the most relevant for fuel pin failure

Inter-granular attack occurs where the steel is sensitized

Such attack on the area of chromium depletion from

a steel layer adjacent to the grain boundaries by

precipitation of carbides at the grain boundaries is

in accord with metallographic observations Opening

of the grain boundaries from the cladding inner

sur-face indicates that attack has occurred along them

In addition, metallic and nonmetallic reaction

products are also detected in the fuel–cladding gap

This indicates that the grains have been chemically

attacked, as evidenced by the roughened surface

The third type of cladding attack is the combined

matrix and intergranular attack that is

characteristi-cally observed in local areas, and is often accompanied

by wastage of the cladding thickness caused by

mechanical interaction or liquid-phase transport of

cladding constituents in the outermost oxide layer

adjacent to the fuel The dissolution of iron,

chro-mium, and nickel in a medium of liquid cesium and

tellurium present in the fuel–cladding gap is known as

cladding component chemical transport (CCCT) It is

interesting to note that the constituents of the cladding

are not uniformly distributed in the reaction zone.17–19

3.16.4 Occurrence of Interaction

Between Oxide Fuels and Cladding

Development

The thermochemistry in the fuel–cladding gap is

complex and is difficult to predict because it depends

not only on concentrations of corrosive FPs, but also

on major parameters such as the fuel–cladding gap

width, fuel O/M ratio, cladding temperature, fuel

temperature, and temperature gradient It is essential

to develop correlations between the loss of claddingstrength and the various parameter groups such asirradiation conditions and fuel specifications Thequalitative characteristics of FCCI and the observedeffects of various fuel and irradiation parameters onFCCI are described next

3.16.4.1.1 Fuel parametersSeverity and frequency of internal cladding attackappear to be independent of both fuel form and fueldensity In the case of vibrocompacted fuel, reduction

in fuel density might contribute to increased FPrelease and to eased radial migration via pores inthe fuel, which has a minor influence on the severity

of the cladding attack (seeChapter2.02, dynamic and Thermophysical Properties of theActinide Oxides)

Thermo-Annular pellet fuel having a theoretical intrinsicdensity of approximately 96%, which corresponds to

a smear density of 80%, showed no clear difference incladding attack in comparison with vibrocompactedfuel of the same smear density.20 From the results, ageneral increasing trend of the depth of claddingattack at cladding temperatures above 500C atapproximately 3 and 5 at.% burnup was indicated inboth pellet and vibrocompacted fuels On the otherhand, Batey and Bagley21 showed that the claddingattack in vibrocompacted fuel was significantly moresevere in comparison with pellet fuels Also, vibro-compacted fuels pins that were irradiated at higherpower levels (57–79 kW m1) showed two to eighttimes the depth of cladding attack expected fromfuel irradiations having the same inner surface clad-ding temperatures.22,23 In contrast, sphere-packedfuel pins which are loaded with spheres of mixed

UO2–PuO2 and UO2 have exhibited decreaseddepths of cladding attack in comparison with pelletfuels with a similar initial O/M ratio and irradiationhistory.24–30Thus, there is no clear explanation of theevidence for different cladding attack behaviorcaused by different fuel forms

From the results of metallographic observations,the influence of the initial O/M ratio on the severity

of cladding attack was emphasized in addition to theinfluence of the cladding temperature,14 and itappeared that the type of the attack was controlled

by the initial O/M ratio The initial O/M ratio has asignificant influence on the depth of claddingattack.31–34 Figure 3 shows the influence of initialO/M ratio on the depth of cladding attack.31,32The effects of initial fuel stoichiometry on the

characteristics of the FCCI have been examined and

the results suggested that hypostoichiometric fuel has

advantages with respect to cladding attack

Further-more, effects of initial O/M ratio on the maximum

depth of cladding attack with increasing burnup have

been confirmed.35

The effects of other parameters could not be clearly

evidenced This suggests that variations of the other

parameters are not so critical However, fuel impurities

such as C, Si, Ni, or halogens might aggravate internal

cladding corrosion either by independent interactions

or through reinforcement of FP attack as catalysts.Therefore, the changes in characteristics and the cor-relation of depth of FCCI have been determined as afunction of the initial O/M ratio

3.16.4.1.2 Effect of temperatureThe changes in characteristics and the correlation ofdepths of FCCI have also been determined as afunction of the cladding inner surface temperature.The experimental data for this and the results show aremarkable increase in the depth of cladding attack

Maximum cladding temperature ( ⬚C)

Maximum cladding temperature ( ⬚F) (a)

of the International Working Group on Fast Reactors, IWGFR/16, Tokyo, Japan, Feb 21–25, 1977; IAEA: Austria, 1977;

pp 43–48, with permission from IAEA.

with temperature above a threshold of about 500C.

Measured depth of cladding attack in specimens of

various irradiation experiments and the results of

statistical analysis as a function of cladding inner

sur-face temperature are shown inFigure 4.32The depth

of cladding attack increases with higher initial O/M

ratio The threshold temperature of the cladding

attack is higher with lower initial O/M ratio There

is general agreement that the temperature threshold

for cladding attack is an inner surface temperature of

tem-perature threshold of cladding attack was identified

as between 450 and 500C, which is consistent with

in-pile tests

Random nonuniform cladding attack has been

observed at all cladding temperatures down to

500C in FR fuel pins Although high temperature

appeared to promote more widespread attack, the

depth of penetration showed no consistent variation

with temperature in the range normally employed in

FRs Certainly, the cladding attack generally increases

with temperature only in the interval of 500–600C;

however, saturation and a decrease of the cladding

attack were observed above 600C.37Figure 5shows

the temperature dependence of depth of cladding

attack and of neutron-induced swelling in stainless

steel cladding used for the fuel pins of Phenix.53The

maximum swelling occurred near the relative axial

position of 0.7 in the Phenix fuel pins Generally,

swelling of FR-MOX fuel increases with burnup andresults in fuel–cladding gap closure.18,54 At furtherburnup, swelling of the cladding begins to occurdepending on the swelling properties of each material,such as length of the incubation period A large fuel–cladding gap forms again, and the FPs are released intothe gap in accordance with the formation of the so-called JOG (joint oxide-gaine in French).18,19,54,55As FPs

in the gap gradually migrate to a colder region of thefissile column, the gap conductance should bedegraded.56,57Therefore, the maximum temperatureincrease occurred across the fuel–cladding gap at near

a relative axial position of 0.7 in the Phenix fuel pin asshown inFigure 5 This large temperature differenceacross the gap would lead to a thermodynamic drivingforce for cladding attack

3.16.4.1.3 Effect of burnupWith an increase in fuel burnup, there is significantgeneration and migration of FPs such as Cs, Te, I, and

Mo In addition, the oxygen potential increases withirradiation in the fuel pin and fuel periphery Thisdetermines the thermochemically stable chemicalreactions which occur between FPs and claddingconstituents A quasilinear relation exists betweenthe maximum cladding depth of FCCI and burnup.However, the influence of burnup on the depth of FPpenetration into the cladding is not clear Table 1

summarizes the characterization of FCCI as a

Experiment Rapsodie I RAPS.-MON.

MFBS 6 DFR 304 DFR 350 DFR 435 MOL 7A MOL 7B

function of O/M ratio, burnup, and cladding inner

surface temperature.6On the basis of the results of

postirradiation examinations,32,46 a slight burnup

dependency on the severity of cladding attack was

obtained from the beginning until approximately

10 at.% (Figure 6)

Sections from irradiated oxide fuel pins showed

no consistent variation in maximum cladding

pene-tration with burnup in the range up to 10.0 at.%, as

shown inFigure 7.41

Perry et al.16and Batey and Bagley21investigated

cladding attack over a wide range of burnups;

how-ever, a clear dependency was not obtained Go¨tzmann

et al.40have evaluated the in-pile data from the

view-point that the mass transport in the reaction layer was

the rate-determining step of the cladding attack, that

is, the corrosion rate depends on the square root ofthe burnup McCarthy and Craig58 have correlatedthe cladding corrosion depths with cladding innerwall temperatures assuming that the cladding corro-sion is proportional to the burnup

3.16.4.1.4 Effect of temperature differencebetween fuel and cladding

With an increase in fuel burnup, the fuel pellet isswollen, and the distance between the fuel and thecladding is generally decreased However, when thecladding diameter increase is larger at high burnup, thedistance between the fuel and the cladding becomeswider again Thus, thermodynamic conditions might

Table 1 Characterization of cladding attack as a function of O/M ratio, burnup, and cladding inner surface temperature

Low (0–3 at.%) Moderate (3–6 at.%) High ( >6 at.%) High (1.98–1.99) >675 Ca intergranular >675 C evolved matrix >500 C evolved matrix

<675 C matrix <675 C matrix <500 C no FCCI

<500 C no FCCI <500 C no FCCI 6 pins/19 samples

7 pins/36 samples b 2 pins/7 samples Moderate (1.96–1.97) >550 C matrix >550 C matrix >650 C combined

<550 C no FCCI <550 C no FCCI >525 C matrix

27 pins/68 samples c 3 pins/11 samples <525 C no FCCI

7 pins/34 samples Low (1.94–1.95) >700 C shallow matrix >625 C shallow

intergranular

>600 C shallow intergranularand matrix

<700 C no FCCI <625 C no FCCI <600 C no FCCI

4 pins/16 samples 3 pins/11 samples 4 pins/16 samples

aCladding inner surface temperature.

bNumber of fuel pins and samples examined with irradiated O/M and burnups.

cIncludes 16 pins/30 samples from HEDL P-15 test.

Source: Lawrence, L A Nucl Technol 1984, 64, 139–153.

Cladding inner surface temperature (K)

900

Phenix clad attack

Figure 5 Comparison of distribution of maximum depth of cladding attack with the profile of neutron-induced

cladding swelling of fuel pin Reproduced from Fee, D C.; Johnson, C E J Nucl Mater 1981, 96, 80–104, with permission from Elsevier.

be rearranged with a widened fuel–cladding gap

because of the notably marked influence of cladding

deformation, in addition to the axial migrations of

FPs and oxygen

The effect of cladding strain on the fuel–cladding

gap widening has been investigated.54,55Figure 8(a)

shows the effect of cladding strain on the depth of

cladding attack with increasing burnup.55 The

oxy-gen potential continues to increase with irradiation at

cladding strain (DD/D) less than 1% The cladding

attack under normal conditions would be controlled

by rather slow corrosion kinetics On the other hand,

at cladding strain (DD/D) beyond 1% cladding strain,cladding attack exhibits much faster kinetics because

of the abundantly supplied reactive agents

As illustrated schematically inFigure 8(b),59 theinterface region is considered as fuel (a) and cladding(b) separated by a gas-filled or reaction product-filledgap which supports the bulk of the temperaturedifference When the swelling of cladding is high

1400

538 Temperature of cladding inside surface ( ⬚C) (a)

Temperature of cladding inside surface ( ⬚F)

649

Figure 6 (a) Measured depth of cladding attack on fuel pins of type 316 (20 % CW) cladding at different local burnup and predicted depth of cladding attack using HEDL correlation 32 Reproduced from Roake, W E.; Hilbert, R F.; Adamson, M G.;

et al In Technical Committee Meeting on Fuel and Cladding Interaction, Proceedings of the International Working Group

on Fast Reactors, IWGFR/16, Tokyo, Japan, Feb 21–25, 1977; IAEA: Austria, 1977; pp 137–158, with permission from IAEA and (b) measured depth of cladding attack as a function of cladding inner surface temperature at different local burnup.46Reproduced from Go¨tzmann, O.; Du¨nner, Ph In Technical Committee Meeting on Fuel and Cladding Interaction, Proceedings

of the International Working Group on Fast Reactors, IWGFR/16, Tokyo, Japan, Feb 21–25, 1977; IAEA: Austria, 1977;

pp 43–48, with permission from IAEA.

and the fuel–cladding temperature gap is large,

clad-ding attack might be more enhanced by different

thermochemical conditions Thus, the temperature

gradient (or lack thereof) between fuel and cladding

is a key parameter determining the driving force and

occurrence of cladding attack The majority of

irra-diated fuel pins retain a significant fuel–cladding gap

over the greater part of the fuel column But with

increasing burnup, this gap is either partly or fullyfilled with reaction products.18,55–61The reaction pro-ducts in the gap are also seen to have extruded downinto radial cracks at the fuel periphery, and in somecases islands of a phase having bright metallic ingotsare contained in the corrosion product The thermal,mechanical, and physicochemical properties of theoxide fuels vary in a continuous manner owing to

Average burnup (at.%)

Fuel pins with strained cladding

Fuel pins with unstrained cladding

B

Figure 8 (a) Effect of cladding strain on depth of cladding attack with increasing burnup Reproduced from Bailly, H.; Menessier, D.; Prunier, C The Nuclear Fuel of Pressurized Water Reactors and Fast Neutron Reactors: Design and Behavior; Intercept Ltd.: UK, 1999; p 475, ISBN: CEA 2-7272-0198-2, with permission from CEA and (b) schematic illustration of temperature gradient in fuel–cladding gap interface Reproduced from Adamson, M G.; Aitken, E A.; Lindemer, T B J Nucl Mater 1985, 130, 375–392, with permission from Elsevier.

the presence of the reaction products, which thus

directly influence fuel temperatures, cladding attack,

and fuel–cladding interaction The physical and

chemical states of the different FP compounds

deter-mine the volume occupied by these species and

there-fore swelling, that is, the volume increase of fuel pellets

resulting from fission

3.16.4.1.5 Effects of cladding materials

Corrosion resistance of three major types of stainless

steel claddings, M316 (austenite), FV548 (ferrite), and

Nimonic PE16 (high Ni), have been investigated in an

irradiation experiment.21The incidence and severity of

cladding attack were almost identical or

indistinguish-able in oxide fuel pins irradiated under similar

condi-tions irrespective of cladding type Furthermore,

corrosion of cladding materials in the fully solution

treated or 20% cold-worked conditions were

investi-gated for M316 and M316L stainless steel cladding

pins The influence of cladding heat treatment on

corro-sion sensitivity was not detected (seeChapter 2.08,

Nickel Alloys: Properties and Characteristics;

Chapter 2.09, Properties of Austenitic Steels for

Nuclear Reactor Applications;Chapter4.02,

Radi-ation Damage in Austenitic Steels;Chapter4.03,

Ferritic Steels and Advanced Ferritic–Martensitic

Steels;Chapter4.08, Oxide Dispersion

Strength-ened Steels; andChapter4.04, Radiation Effects in

Nickel-Based Alloys)

The corrosion susceptibility of M316 stainless

steel (carbon content0.05%) and M316L stainless

steel (carbon content 0.02%) was also investigated.21

Although carbide precipitation and associated

chro-mium depletion at grain boundaries should facilitate

intergranular attack, it was indicated that the initial

carbon content was not dominant among the factors

determining corrosion resistance Although

carburiz-ing or nitridcarburiz-ing of the claddcarburiz-ing by transfer of carbon

and nitrogen from the fuel occurred, neither

promo-tion nor prevenpromo-tion of FCCI was shown.21

On the contrary, out-of-pile results showed that

carbon may influence the nature of FP-induced

clad-ding attack by saturating alloy grains near the

cladding surface in the form of intergranular carbide

precipitates.62Heavily carburized regions of the

clad-ding material were chemically attacked by Cs–Te

mixtures; the result was an attack zone with a more

uniform appearance than the deep intergranular type

observed with alloys of large grain size

The effects of nitriding of cladding materials on

the attack were identified in an irradiation

experi-ment.8The cladding was irregularly nitrided on the

inner surface to a maximum depth of 4 mil (102mm)because of fuel pellets in the top half of the fuelcolumn which each contained 4800 ppm of nitrogen.Cracks in the cladding were occasionally observed in

or near the nitrided layer, and they were caused bythe differential expansion of this layer However, pre-cipitates were observed along the cracks betweenthe nitrided layer and the remainder of the cladding,which were suggested to be chromium nitrides.Although nitriding of the cladding materials wouldappear to complicate the situation of cladding attack,the corrosion resistance of steels should be decreased

by the precipitation of chromium.63

Several mechanism models have been proposed toexplain FCCI in MOX fuels based either on oxidation

or materials transport processes However, the fuel pindesign only varies with respect to empirical equationsrather than fundamental models because explanations

of the observed effects of FCCI are speculative fore, FCCI wastage equations for design have beenbased on collected in-pile test data Wastage correla-tions for fuel pin design were developed by relating theloss of effective cladding thickness to irradiation andfabrication parameters of the fuel pins The followingwastage correlations, expressed using penetrationdepth, have been proposed

There-The ANL (Argonne National Laboratory) tion, which was made by fitting the HEDL (HanfordEngineering Development Laboratory) data, wasdeveloped in order to evaluate in detail the relativeimportance of the temperature difference across thefuel–cladding gap The average depth of claddingattack was obtained by the following correlation:Ave:WDepthðANL; mmÞ ¼ 3  105

correla-B½4:4  104exp 31200=TðfuelÞ

 2:7  103exp 24600=TðcladdingÞ
 ½1where B¼ local burnup (at.%), T(fuel)¼ temperature

of fuel outer surface (K), and T(cladding)¼ temperature

of cladding inner surface (K)

Another wastage correlation developed at ANL64isshown ineqn [2]and is based on EBR-II (experimentalbreeder reactor-II) and FFTF (Fast Flux Test Facility)data The EBR-II data included 305 data sets from

104 fuel pins with Type 316 stainless steel claddingand 24 data sets from ten fuel pins with D9 stainlesssteel cladding, irradiated to approximately 17 at.%burnup at T values up to 730C FFTF data included

78 data sets from a mixed total of 24 pins irradiated

to about 15 at.% burnup at T values up to 685C

WDepthðANL; mmÞ ¼ 0:5070  ðO=M  1:935Þ

Here, O/M> 1.935, B > 0, and T > 705; B ¼ local

burnup (at.%) and T¼cladding inner surface

temper-ature (K) The standard deviation of the correlation

was 12.5mm

A third correlation was obtained from an

experimental relationship for a low-burnup fuel pin

(1.2 at.%), designated P-23A EBR-II The average

depth of cladding attack yieldedeqn [3]65:

Ave:WDepthðHEDL;mmÞ ¼ 3:36  105expð10063=TÞ ½3

where T¼cladding inner surface temperature (K)

Nonlinear regression analysis of the data from all

three fuel pins of two interim examinations of P-23A

HEDL fuel pins at 2.4 and 5.0 at.% yieldedeqn [4]66:

Ave:WDepthðHEDL; mmÞ ¼ 2:43  106B0:517

where B¼ local burnup (at.%) and T ¼cladding

inner surface temperature (K)

A later wastage correlation67based on EBR-II data

is shown ineqn [5]:

Max:WDepthðHEDL;mmÞ

¼ 0:4521ðB þ CÞ  ðO=M  1:942ÞðT  728Þ ½5

for B 0, O/M > 1.942 and T > 728; for conditions

outside these ranges, depth¼ 0 Here, B ¼ local

burnup (at.%), C¼ 12.23, O/M ¼ initial O/M ratio,

and T¼local time-averaged temperature of cladding

inner surface (K)

One GE (General Electric Company) correlation

of the cladding attack based on 68 irradiated fuel pin

sections was developed and included in fabrication

and operating parameters.4,66,67 This correlation is

given aseqn [6]:

Max:WDepthðGE; mmÞ ¼ 2:36  108

tðO=M þ 0:001B  1:96ÞQ2:33ð1  pÞ

where T¼cladding inside surface temperature (K),

t¼ equivalent full power days of irradiation, O/M ¼

initial O/M ratio, B¼ local burnup (at.%), p ¼ smear

density (a fraction of the theoretical density), and

Q¼ peak linear heating rate (W mm1)

Another GE model involves CCCT as shown in

eqn [7] This GE (CCCT) model applies only to the

cases that high initial O/M≧ 1.98 and high burnup ≧

5 at.% are involved:

Max:WDepthðGEðCCCTÞ;mmÞ ¼ 4:14  105B0:5

expð9776=TÞ þ 0:86ðT  873Þ0 :5ðB  4Þ ½7where B¼ local burnup (at.%) and T ¼claddinginner surface temperature (K) In this equation, thefirst term represents the conventional (oxidative)FCCI and the second term represents the contribu-tion from CCCT

The SNR (Schneller Natriumgeku¨hlter Reactor) relation52 which was based on DFR (Dounreay FR)and Rapsodie (French experimental FR) data isshown in eqn [8] The correlation fits the averagecorrosion depth and emphasizes the influence of theinitial O/M ratio on the corrosion depth (i.e., a strongincrease for O/M between 1.96 and 1.98)

cor-Max:WDepthðSNR;mmÞ ¼ 96:97½1  3:013  105ð2  O=MÞ4  exp½76:920=ðT  769Þ ½8for 1.96 O/M 2.00 and 769 T 923 K; T ¼cladding inner surface temperature (K) and O/M¼initial O/M ratio

The PNC (Power Reactor and Nuclear FuelDevelopment Corporation) correlation which sug-gests the maximum cladding corrosion depths forthe fuel pin design is shown ineqn [9]41:

Max:WDepthðPNC; mmÞ ¼ 4:64  104

 ðO=M  1:93Þ  ðT  360Þ  B ½9where T¼ temperature(C), O/M¼ initial O/Mratio, and B¼ burnup (MWd t1); but T¼ 620(if cladding temperature 620), B ¼ 35 000 (ifburnup35 000), and O/M ¼ 1.98 (if initial O/Mratio1.98)

A wastage correlation developed at PFR type FR),68 which was based on data (up to 10 at.%burnup), is shown ineqn [10]

(proto-Max:WDepthðPFR; mmÞ

Relatively conservative wastage correlations [2], [5],and [9] that employed major parameters such as clad-ding temperature, burnup, and O/M ratio are like theenvelopes of measured data points Those parametersare conservatively considered relating to thresholdcladding temperature, incubation period (burnup),and threshold oxygen potential (O/M ratio) Thewastage correlation (10) was developed on the basis

of experimental data, but it conservatively includes a

contribution from fabrication tolerances and external

corrosion (as well as FCCI)

Wastage correlations [1], [3], [4], and [6–8]

obtained by fitting experimental data with major

para-meters such as cladding temperature, burnup, and

O/M ratio were developed on the basis of the

conven-tional (oxidative) FCCI The form of the correlation

equations were mainly expressed as an exponential

temperature relationship Thermally activated

pro-cesses assumed in cladding attack exhibit exponential

temperature dependence In addition, the wastage

cor-relation [7] involves the term of CCCT contribution

expressed by threshold values (cladding temperature

and burnup) The wastage correlation [6] includes a

parameter of the smear density that has a large impact

on the fuel behavior, because of its large influence on

the thermal conductivity of the fuel Although much

data is collected for FCCI, the range of parameters and

the uncertainty of the cladding temperature constitute

serious obstacles to developing consistent wastage

correlations The form of correlations, except the

correlation [7], was not chosen to reflect current

knowledge of probable FCCI mechanism The

corre-lations have not successfully predicted and consisted

of actual corrosion behavior Cladding wastage

equa-tions are often used to predict the overall condition of

cladding attack in fuel pins at each burnup level The

current model used in cladding wastage equations

cannot sufficiently account for localized deep attack

Cladding attack depends on various corrosion

mechanisms, reaction potentials, FP transport (release,

radial and axial migration), and condensation within

the fuel–cladding gap, which is always changing during

the course of an irradiation; thus severity of cladding

attack is not directly dependent on the above major

parameters in the correlations The kinetics of FCCI

in fuel pins during irradiation is determined through

consecutive processes: (a) generation and release of

corrosive FPs and supply of free oxygen, (b) transport

of FPs and oxygen to the cladding, and (c) reaction

between the elements and cladding constituent

ele-ments The process (c) is strongly dependent on

clad-ding temperature The process (b) is related to not

only fuel parameters and irradiation parameters but

also nonuniform temperature within the fuel–cladding

gap In addition to change in cladding temperature

during irradiation, the nonuniform temperature

asso-ciated with fuel behavior produces a change in the

character of FCCI The amount of cladding wastage

will be different at each burnup level because of

rela-tive shifts in the processes

3.16.5 Mechanism of Oxide Fuel and Cladding Interaction

3.16.5.1 Oxygen Potential of Irradiated FuelThe occurrence of FCCI depends on the excess oxy-gen available for interaction and on the cladding innersurface temperature The available oxygen is related

to the initial fuel O/M ratio and burnup Thus, theincrease of the oxygen potential ðDGO2¼RTlnpO2Þdue to burnup enhances the rate of the claddingattack.50,53The chemical state of the FPs depends onthe oxygen potential of the fuel, a potential which inturn varies during irradiation, depending on theamount of oxygen consumed by the FPs and therefore

on the nature of the compounds that they form FCCI

is thermochemically controlled mainly by the oxygenpotential (see Chapter 2.02, Thermodynamic andThermophysical Properties of the Actinide Oxi-des; and Chapter 2.21, Fuel Performance of FastSpectrum Oxide Fuel)

In comparison with the fission of uranium, nium fission makes more oxygen available because ofhigher yields of noble metals and lower yields ofoxide formers.39,49The different fission yield curvesfor plutonium and uranium result in greater increases

pluto-in oxygen potential with burnup for containing fuels.69Soon after reactor startup, beforesignificant amounts of FPs have been generated, theoxygen in the fuel redistributes rapidly to make theperipheral fuel nearly stoichiometric Different tech-niques for direct measurement of the radial oxygenprofiles have been employed in postirradiation exam-inations.67–76Figure 9(a)compares the oxygen profile

plutonium-of irradiated samples obtained by an EMF method72with similar profiles, which were obtained by usingdifferent techniques (EPMA analysis of Mo andMoO2 content in metallic and ceramic inclusions74and X-ray measurements of lattice parameters75) Inthe figure, three different zones may be postulated toexist.72Zone a is near the central hole, in which rapiddepletion of oxygen occurs; this zone coincidesapproximately with the columnar grain region Zone

b is an intermediate flat zone (or plateau) Zone c isnear the periphery of the oxide pellet in which theO/M approaches the stoichiometric value

A slight amount of carbon in MOX fuel is included

as an impurity during the fabrication process It waspostulated that the oxygen potential at any point in afuel pin during use is controlled by a local equilibriumwith a CO/CO2 mixture of constant composition,77from which it follows that redistribution of oxygenshould occur to give a nearly stoichiometric fuel at

the fuel periphery Therefore, the oxygen potential

of the fuel periphery in contact with the cladding has

the same stoichiometry in irradiated oxide fuels,

despite the different initial O/M ratios, after an

equil-ibration period of oxygen redistribution As a

conse-quence, the influence of the initial stoichiometry on

cladding attack is not strong From the data of Rand

and Markin,77this stoichiometry at the fuel periphery

corresponds to an oxygen potentialDGO2 in the fuel–

cladding gap of approximately 418 kJ mol1 at

700C, which is more than sufficient for growth of a

Cr2O3-type protective oxide on the inner cladding

surface (the oxygen potential DGO2 in equilibrium

with chromium and Cr2O3at 700C is576 kJ mol1)

The oxygen potential of irradiated Phenix fuel with an

initial composition Pu0.2U0.8O1.982was measured78and

the results are shown inFigure 9(b) TheDGO2 value

increased with increasing burnup from 3.8 to 11.2 at.%,

and it also increased linearly with temperature As for

oxygen stoichiometry of these irradiated fuels, the

DGO2 data measured for the pellet periphery

was regarded as giving the oxygen potential of the

-near-stoichiometric composition.72,78

In addition, for MOX fuel containing small

amounts of americium, the oxygen potential increases

with increasing americium content.79In the Superfactexperiment, four oxide targets containing high and lowconcentrations of237Np and241Am were irradiated inthe Phenix reactor.80,81The maximum depth of corro-sion was 50mm in the SF13 (U0.74Pu0.24Np0.02O2  x)and SF16 (U0.74Pu0.24Am0.02O2  x) fuel pins.The depth of the cladding attack did not exceed thelimit set for standard MOX fuel pins with the sameburnup

3.16.5.2 Characteristics of MajorCorrosive FPs

The chemical state of the corrosive FPs depends onthe oxygen potential of the fuel Oxygen potentialvaries during irradiation, depending on the amount ofoxygen consumed by the FPs and on the nature of thecompounds that they form

Cesium, tellurium, and iodine are often calledvolatile FPs These elements and part of the com-pounds they form are gases at the fuel temperatures.Therefore, they will undergo considerable radial andaxial migration to colder temperature regions Theygenerally accumulate in the gap; in some cases, thisresults in contact between fuel and cladding

Pu/(U + Pu)

2.00

(b) (a)

-700

-160 -140 Burnup, at.%

3.8 7.0 11.2

-120 -100 -80 -300

3.4

BU, at.%

1.99 Final (O/M)

Figure 9 (a) Comparison of present profile obtained with EMF measurements with experimental results of Kleykamp and Conte et al using the Mo/MoO 2 method and X-ray techniques, respectively Reproduced from Ewart, F.; Lassmann, K.; Matzke, Hj.; Manes, L.; Saunders, A J Nucl Mater 1984, 124, 44–55, with permission from Elsevier (b) Oxygen potential measurements with the EMF cell on irradiated Phenix fuel of initial composition (U 0.8 Pu 0.2 )O 1.98 Reproduced from Matzke, Hj.; Ottaviani, J.; Pellottiero, D.; Rouault, J J Nucl Mater 1988, 160, 142–146, with permission from Elsevier.

The cesium is obtained in high yield The large

excess of cesium over I and Te guarantees that not

only almost all the iodine and tellurium are bound as

CsI and Cs2Te, but also that there is still a large

amount of cesium left in the fuel pins In FRs, the

Cs/I and Cs/Te ratios are approximately 10 and 4,

respectively.82The other reactive FPs have very low

yields, and hence it is considered that cesium/oxygen

reactions are dominant in the pins

As shown in many postirradiation investigations,

FP-enhanced oxidation is the main cause of cladding

attack in oxide fuel pins Various out-of-pile

experi-ments have shown that the FPs (I, Te, Sb, Cd, In, and

Sn) directly attack stainless steel.83 Understanding

the dependence of those reactions on the cladding

temperature and the oxygen potential in the system is

important to obtain a better understanding of the

reaction possibilities of the various FPs

3.16.5.2.1 Iodine

Investigations with iodine revealed that reactions

with stainless steels take place at 400C Iodine reacts

predominantly with Cr Preferential reactions along

the grain boundaries of the cladding material yield a

type of reaction similar to that of pitting corrosion.83

However, iodine is not present in elemental form, but

as CsI molecules which are thermodynamically stable

under the conditions in the fuel pins The

equilib-rium pressures of I2 and I in the fuels for various

oxygen potentials are very low and much lower than

the CsI pressure (5107atm at 727C).84

In addition, dissociation of CsI in the strong

G-radiation field of the fuels has been postulated

Cubicciotti and Davies85have shown that free iodine

was released from thermochemically stable solid

iodides by G-radiation The chemical activity of

iodine released into the fuel–cladding gap was

asso-ciated with the amount of cesium which probably

could control the activity

Iodine in the elementary state causes severe attack

on stainless steel cladding However, when bonded

to Cs, it is not corrosive to the cladding materials

Nevertheless, the state of iodine in the fuel has been

questioned for a long time in view of the dissimilar

transport of cesium and iodine in fuels,13 and their

different release rates.86 Investigations of 129I and

137

Cs radial profiles in light water reactor (LWR)

pin sections indicated that iodine migrates slightly

faster than Cs This would contradict the possibility

for excessive CsI formation in the fuels However,

experimental evidence from release characteristics

and postirradiation investigations87 has provided

support for the presence of CsI, rather than elementaliodine, in the fuel pin As other iodides, for instance, of

Zr, Mo, or Ru are less stable,88it can be assumed thatCsI is the dominant iodide species in the fuel pin, andthat it will escape as such when lack of cooling causesthe fuel temperature to rise Finally, as the atomicratio Cs/I is approximately 10, it can be expectedthat all, or almost all iodine is present as CsI.Hofmann and Go¨tzmann83 performed out-of-pilereaction tests of cladding material with UO2and UO2.08

in the presence of CsI or iodine High-purity CsI didnot react with the cladding steels even at 800C for

1000 h Nevertheless, reactions took place with thecladding when the oxygen potential was above a criticalvalue The maximum depth attained by the reactionwith stoichiometric UO2 was about 10mm at somepoints No reaction between CsI (simulated burnup

20 at.%) and stoichiometric UO2at 800C for 1000 hwas detected; however, marked reactions took placewith UO2.08 (reaction depth, about 20mm) On theother hand, reactions with UO2.08were much reduced

in the absence of CsI The addition to UO2.08of freeiodine caused reactions up to 50mm in depth in thecladding material after 1000 h at 800C This indicatedthat the oxidation of the cladding by hyperstoichio-metric fuel was considerably accelerated in the pres-ence of CsI and I

3.16.5.2.2 Cesium

A large excess of cesium over I and Te is generated byfission Although almost all iodine and tellurium arebound as CsI and Cs2Te, there is still a large amount

of cesium left in the fuel pin Elemental cesium has ahigh volatility, but formation of cesium uranate orcesium molybdate lowers and determines the vaporpressure of cesium

Hofmann and Go¨tzmann83 conducted out-of-piletests of cladding material with UO2 and UO2.08 inthe presence of Cs Elemental cesium was compatiblewith stainless steels up to 3000 h at 1000C Even minorimpurities of cesium with oxygen caused a reaction withthe cladding The depth of cesium reactions showed adependence on the O/M ratio of the fuel The claddingattack by hyperstoichiometric fuel was considerablyaccelerated in the presence of cesium While after

1000 h, reaction zones of less than 5mm were observed

in contact with UO2.08at 800C, 100mm was attainedafter the addition of Cs (simulated burnup 10 at.%) Ifmolybdenum was added to the mixture of UO2.08þ Cs,the reactions with the cladding became weaker.The Cs compound Cs2CrO4 reacted very vio-lently with stainless steels at 800C and 1000 h and

grain boundary reactions took place, penetrating

more than 1000mm Cs2Cr2O7also reacted with the

cladding The chemical interactions covered some

50mm at 800C and 1000 h.83

Formation of various cesium compounds is

deter-mined by the oxygen potential This indicates that

reactions between cesium compounds and cladding

occur only above a certain oxygen potential The

change in chemical constitution within the gap with

variation of oxygen potential of a fuel (U0.7Pu0.3O2 )

in the presence of FPs (simulated burnups 2, 10,

and 20 at.%) has been examined by thermochemical

calculations of thermodynamic equilibria.89 From

thermodynamic studies, formation of the major cesium

compounds was determined at certain oxygen

poten-tials of a fuel The reactions which can buffer the

oxygen potential within the gap are given inTable 2

Formation of cesium uranate and molybdate

was thermodynamically suggested.83 Postirradiation

examinations of highly irradiated fuel showed

Cs2MoO4to be present on the fuel side of the fuel–

cladding gap By contrast, the formation of Cs2U4O12

instead of Cs2UO4or Cs2UO3.5was observed in

irra-diated fuel.13The formation of this uranate will cause

considerable swelling of the fuel90,91and decrease the

fuel–cladding gap width

3.16.5.2.3 Tellurium

Tellurium is a precursor of iodine TeO2 cannot

be formed because the oxygen potential of the

Te/TeO2 system is much higher than that inthe fuel Formation energy of stable Cs2Te appears

to be approximately 376 kJ mol1, which is justenough for Cs2Te to be stable at the conditions inthe fuel Cs2Te showed an appreciable volatility

at the conditions in a fuel.92 The vapor pressure

of Cs2Te is about 10–4atm at 727C, which meansthat an appreciable amount of tellurium is alreadypresent as Cs2Te in the gap at normal operatingconditions of the fuel pins If gas-phase transport

is the main path for FPs to reach the fuel surface, ashas been stated,93it is important to know the vaporpressure of Cs2Te

Pure tellurium showed marked penetration instainless steel up to depths comparable with thoseobserved in irradiated fuel pins at temperaturesbelow 800C Furthermore, out-of-pile experimentsshowed that the reaction of tellurium does notdepend on the oxygen potential unlike in the case

of cesium.94The reactions with tellurium are clearlyfunctions of irradiation period and temperature.Go¨tzmann and Hofmann95determined the maxi-mum penetration depths of tellurium in stainlesssteels as a function of the temperature and annealingperiod At 500 and 700C, attack occurred along thegrain boundaries of the cladding Below 500C auniform attack was observed, and tellurium migratedinto the cladding by interfacial reactions Above

700C, the penetration depths of tellurium weredependent on the grain size of the cladding material

Table 2 The reactions which can buffer oxygen potential within the fuel–cladding gap of fast reactor fuel pins

phases present

900 K 1200 K

1 654 586 4Cs þ 2½U; PuO2x þ 3 þ x
O 2⇄2Cs2½U; PuO3:56 CsI, Cs 2 , Te, Mo, Cr

Cs 2 Te, Mo

6 549 494 2Mo þ 2Cs2½U; PuO3:56 þ 5 þ x
O2⇄2Cs2MoO4 þ 2½U; PuO2x CsI, Cs 2 Te, Cr 2 O 3

7 525 351 2Cs2½U; PuO3:56 þ 1 O2⇄2Cs2½U; PuO4 CsI, Cs 2 MoO 4 , Cs 2 Te,

Cr 2 O 3

Cr 2 O 3

10 312 266 Cs2Te þ ½U; PuO2x þ ð1 þ x=2ÞO2⇄Cs2½U; Pu4 O12 þ Te CsI, Cs 2 MoO 4, Cs 2 Te,

Cr 2 O 3

12 219 180 2Cs2MoO4 þ 4½U; PuO2x þ 1 7  x
O2⇄Cs2Mo2O7 þ Cs2½U; Pu4 O12 CsI, Te Cr 2 O 3

Source: Ball, R G J.; Burns, W G.; Henshaw, J.; Mignanelli, M A.; Potter, P E J Nucl Mater 1989, 167, 191–204.

The maximum penetration depth of tellurium was

greater in coarse grains than in fine grains

When the oxygen potentialDGO2is approximately

376 kJ mol1, Cs2Te is stable in the fuel When the

oxygen potential reaches a sufficiently high level

(>418 kJ mol1), Cs2Te becomes less stable than

cesium molybdate or urinate.96 Cs2Te can attack

the cladding by reaction with the cladding

constitu-ents, forming cesium chromate, iron telluride, and

nickel telluride Because of the dissolution of the

cladding materials into Te, the cladding constituents

diffuse into the fuel even at temperatures as low as

400C In contrast, tellurium diffuses into the

clad-ding material and is found at the reaction front

together with Cr.96

However, the highly localized concentration of

tel-lurium required for cladding attack in FR fuel pins

appears to be inconsistent with the postirradiation

investigations of numerous cladding attack regions in

which tellurium has not been detected.37,39,40,46 The

role of tellurium in cladding attack is largely based on

results from out-of-pile tests38,40,61,94,97–103 in which

the average tellurium content per unit area of cladding

inner surface was much greater than that encountered

in irradiation tests

3.16.5.3 Various Corrosion Reaction

Mechanisms

3.16.5.3.1 Corrosion early in life

Generally, postirradiation examinations of specimens

irradiated at low power (20 kW m1) and low

burnup (2 at.%) have shown the absence of fuel–

cladding chemical attack.47,104At low burnup, a radial

redistribution of oxygen occurs early in life Oxygen

migration down the temperature gradient provides

oxygen activity at the cladding surface sufficient to

oxidize the Cr in the cladding material Significantcladding attack is not expected until FPs (Cs, Te,and I) are available to participate in an enhancedcorrosion process The quantities required are small,however, and sufficient quantities are generated

by approximately 0.4 at.% burnup to initiate thereactions Therefore, it is expected that even at burn-ups as low as 1 at.%, significant reaction can beobserved.55An internal cladding corrosion phenome-non due to Te and I has been observed,7which is linked

to the oxide thermal behavior at the very beginning oflife This intergranular type of corrosion is very deepand it is characteristic of pins having been operated athigh linear power at the very beginning of irradiation.The fission yield for cesium is generally higherthan is needed to react with Te and I Potentials

of tellurium and iodine are greatly reduced unlesscesium is consumed by other reactions

Figure 10 shows that tellurium-induced grainboundary cladding attack in short-term irradiationtests (burnup 0.1 at.%) of annular fuel occurred in alow density fuel pellet of 86% T.D (theoretical den-sity) but not in a higher density fuel.7The maximumpenetration depth of this type of cladding attack was

60mm after 11 days of irradiation Similar localizedintergranular cladding attacks are likely to appear atthe very beginning of life (from loading to an irradia-tion of 10 days) and show very deep penetrations Thisvery beginning of life corrosion, termed ‘corrosion-de-jeunesse’, in French, was in the form of spots of limitedarea in fuel pins irradiated at high linear heating rate.55This cladding attack is caused by corrosive FPs(I and Te) accumulating in contact with the cladding.Iodine and tellurium formed by fission generate Csand Rb through radioactive decay after roughly

10 days Tellurium could rapidly form compoundssuch as Cs2Te However, because of the time required

Figure 10 Tellurium-induced early-in-life cladding attack (burnup 0.1 at.%): (a) chromium oxide scale on cladding wall and (b) intergranular attack with grain boundaries ( 60 mm) Reproduced from Go¨tzmann, O In Fast Reactor Core and Fuel Structural Behaviour, Proceedings of the International Conference, Inverness, June 4–6, 1990; BNES: London, UK, 1990;

pp 1–8, with permission from BNES.

for the formation of rubidium and cesium, the iodine

and tellurium elements are in excess at the very

beginning of irradiation Under the influence of

high linear heat rating, these FPs migrate radially

and axially to the colder temperature regions If, in

these regions, the oxygen potential and the cladding

temperature are high enough, significant

intergranu-lar attack of cladding materials develops

3.16.5.3.2 Iodine transport of cladding

constituents

Free iodine reacts with cladding materials But

iodine is easily bound to Cs during irradiation, so

only insignificant quantities of I are available to react

with cladding materials A reaction of the cladding

materials with I needs a sufficiently high partial

pres-sure of I which is not possible from a thermodynamical

viewpoint However, metallic inclusions of pure iron

from the cladding by iodine transport were observed

within the irradiated fuel in addition to intergranular

and matrix attack of cladding.16,105–107 Figure 11(a)

shows the appearance of metallic precipitates along

fuel cracks observed in irradiated MOX fuel.105

The radiation effect on the partial pressure of I,

involving the effect of recombination, has been

evaluated for gaseous CsI based on kinetic theory.108The calculations were carried out for an oxygenpotential DGO2 ¼ 418kJmol1 at the claddinginner surface temperature of 600C The partialpressure of iodine under the radiation condition wasapproximately 102Pa (107atm), which washigher than that under the nonradiation conditionapproximately 109Pa (1014atm) Radiationeffect on the partial pressures of FeI2, CrI2, andNiI2as a function of cladding temperature is shown

inFigure 11(b).108These calculations indicated thatvapor transport of iron and chromium was possiblewithin a FR fuel pin And the pressure of NiI2(underthe radiation condition) was so low that significanttransport may not occur

Aubert et al.107proposed a vapor transport nism by iodine which is similar to the Van Arkel–deBoer process According to the vapor transportmechanism, iodine reacts with stainless steel at thecold region of the fuel–cladding gap to form metaldiiodides and they diffuse through the gap into thefuel The radiation-induced decomposition of metaldiiodides occurs and metal iodides are formed at thefuel surface which corresponds to the hot regions.This leads to transportation and precipitation of

mecha-800 (b) (a)

500 600 700

Figure 11 (a) Appearance of ‘metallic rivers’ observed in MOX fuel The rivers apparently originated at the fuel–cladding interface Reproduced from Fitts, R B.; Long, E L., Jr.; Leitnaker, J M In Fast Reactor Fuel Element Technology, Proceedings of Conference, New Orleans, LA, Apr 13–15, 1971; Farmakes, R., Ed.; American Nuclear Society: Hinsdale, IL, 1971; pp 431–458, with permission from ANS (b) Radiation effect on the partial pressures of Fel 2 , Crl 2 , and Nil 2 as a function

of cladding temperature Reproduced from Konashi, K.; Yano, T.; Kaneko, H J Nucl Mater 1983, 116, 86–93, with permission from Elsevier.